The instant invention relates to a process for making a matrix of thin layer transistors and, more particularly, to a matrix of thin layer transistors with memory capacitors, especially for a liquid crystal display screen.
A process for making a matrix of a-Si:H-thin layer transistors with memory capacitors is currently known and described in German Published Patent Application 43 10 640.4. This process requires only three photolithographic masks and transistors of higher mobility are obtained from it. In the known manufacturing process a layer sequence comprising an insulating layer, a-Si:H layer acting as an undoped semiconductor layer, a doped semiconductor layer and a conductive layer is provided having the layers one following the other without intervening structures between the individual layers and is subsequently structured in two masking operations. In this method the conductive layer, which subsequently serves as a mask for etching of the doped and undoped layers, is first structured first. The end of the etching process of the semiconductor layers may be controlled by controlling the optical emission of the etching plasma. The doped semiconductor layer is however etched only in the vicinity of the semiconductor layer ducts, also on a surface area, which amounts to less than 0.5% of the total substrate surface area exposed to the etching plasma. The optical signal, which is produced by this reduced surface area in the etching plasma, is reduced in intensity to such an extent that determination of the end point of the etching process of the doped semiconductor layer leads to great difficulties. This leads frequently to overetching, chiefly the intrinsic semiconductor layer under the doped semiconductor layer is etched more rapidly than the doped layer.